Continuously variable magnification (reduction) systems find important application in photocopying systems such as those discussed in the above-mentioned patent applications. Since cost is an everpresent factor in such systems, it is more desirable to use a single-focus lens than a variable-focus lens if possible. Also, for cost reasons, it is desirable to use single-focus lens with maximum acceptable tolerances on the total conjugate length (TCL). As the tolerance is lessened, the expense of the lens is considerably increased. Therefore, in order to realize the economies of utilizing a single-focus lens over a variable-focus lens, it is necessary to provide a system in which the loss of focus due to lens tolerance is kept to acceptable levels while still using a relatively high tolerance lens.
For a continuously variable reduction system used in an electrophotographic copier machine such as described in the above-referenced patent applications, it has been found that when a single-focus lens is adjusted to provide an in-focus condition at a 1:1 copying ratio, the focus error becomes unacceptably large as the reduction ratio is increased for lenses at tolerance limits of, for example, .+-. 1%. It has been found that for a machine designed to produce a 0.647 reduction ratio, it is necessary to go to a .+-. 0.5% tolerance lens in order to provide acceptable focus error. In this example, tolerance is measured on total conjugate length (TCL). Such a lens increases the cost of the copy machine considerably.
In a system in which a single-focus lens is cam driven to the desired reduction ratio, one solution is to provide a customized cam for each lens; obviously, such a solution is expensive. An alternative solution is to require, for example, 3 cams -- one set of cams for a lens within a certain specification tolerance range, a second set of cams for a second range of tolerances, and a third set of cams for the final range of tolerances. This solution requires the checking of every lens to place the lens within a specific tolerance range and multiplies the inventory on parts. Consequently, this solution is also expensive and less reliable since it is subject to manufacturing error. It is, therefore, the primary object of this invention to provide a method of adjustment and a simple, inexpensive apparatus to retain the economic plus of a single-focus lens system while maintaining acceptable focus and magnification tolerance in a continuously variable reduction system.